The invention concerns a controller for shifting a gearbox the driveline of an internal combustion engine, in particular a power shift stage, whereby shifting between rotational speed ratios is accomplished in dependence upon engine load.
A power shift stage is widely used in agricultural tractors. The power shift stage is usually installed between a clutch and a conventional multi-speed gearbox. It is a planetary gearset that can be shifted hydraulically under load, which transmits the engine speed either unchanged or reduced by a certain amount, for example 20%. With the reduction in engine rotational speed the torque transmitted to the driving wheels is increased.
In a first shift position for normal vehicle speeds, hydraulic actuation of a multi-disk clutch connects the planet carrier rigidly with an inner sun gear. The planet carrier thus rotates with the sun gear. As a result the input and output shafts of the power shift stage rotate at the same speed.
In a second shift position the planet carrier is stopped relative to the gearbox housing by means of a hydraulically operated multi-disk brake. The power flow now runs through the sun gear which is connected to the input shaft, to the planet where a speed reduction occurs, to the second sun gear which is connected to the output shaft of the power shift stage. Hence the output shaft speed of the power shift stage is reduced from the input shaft speed. Simultaneously the output torque is increased.
The second shift position of the power shift stage is applied when an increased torque is demanded by the drive. The shift between the two shift positions is possible without interruption of the power flow between the engine and the drive wheels.
The shifting of the power shift stage is usually performed by manual operation of a shift lever In the operator's cab, by means of which a hydraulic valve can be controlled. The hydraulic valve selectively directs pressurized oil to the multi-disk clutch and to the multi-disk brake of the power shift stage.
A shift from the first to the second shift position should then be performed when the load on the drive increases (for example, due to an increase in slope of the terrain traversed or a change in the soil condition) to such a degree that the rotational speed of the engine is reduced and drops below the region of maximum power output. The shift accomplishes an increase in the rotational speed of the engine with constant vehicle speed. By this means the productivity of the drive is improved and the torque reserve increased considerably.
Upon reduction of the load on the drive a shift from the second into the first shift position should occur when the rated speed of the engine is exceeded and a sharp drop in the power output results as the power curve falls off. Otherwise the output of the engine falls off sharply and the specific fuel consumption increases by leaps and bounds.
The shift points are established by the operator on the basis of experience. However the shift from the second into the first shift position usually occurs at too high an engine speed. This may result in stable driving conditions, but the engine no longer operates at maximum possible power. Furthermore, the fuel consumption increases.